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Regions are the largest watersheds shown. Regions contain either the drainage areas of a major river, such as the Missouri region, or the combined drainage areas of several rivers, such as the Texas-Gulf region. They were designated by the U.S. Water Resources Council and are used for comprehensive planning. Regions are differentiated by color in the National Atlas Map Maker. The regional hydrologic unit name is indicated on the Hydrologic Units Wall Map.

Subregions divide the regions and they include the area drained by a river system, a section of a river and its tributaries in that reach, a closed basin(s), or a group of streams forming a coastal drainage area.

Accounting units subdivide many of the subregions. They are used by the USGS for managing national water data.

Cataloging units are the smallest hydrologic subdivisions shown. Most are larger than 700 square miles in area. A cataloging unit is a geographic area representing part or all of a surface drainage basin, a combination of drainage basins, or a distinct hydrologic feature.

Regional and subregional boundaries serve as accounting and cataloging unit boundaries.

A hydrologic unit code is an eight-digit number that identifies each of the watersheds into which the country has been divided for the purpose of water-resources planning and data management. The code uniquely identifies each of the four levels of hydrologic classification within four two-digit fields.

The first two digits identify the water-resources region; the first four digits identify the subregion; the first six digits identify the accounting unit; and the addition of two more digits identifies the cataloging unit.

On the National Atlas Hydrologic Units Wall Map, the code is shown as a group of six digits and a separate group of two digits. This example shows how to identify, by hydrologic unit code, the sample cataloging unit shown below.